5-alpha reductase is an enzyme that converts testosterone, the male sex hormone, into the more potent dihydrotestosterone:
Note the major difference -- the Δ4,5 double-bond on the A (leftmost) ring. (The other differences between the diagrams are unrelated to chemical structure.)
There are two isoenzymes, steroid 5-alpha reductase 1 and 2 (SRD5A1 and SRD5A2). The second isoenzyme is deficient in 5-alpha-reductase deficiency which leads to a form of intersexualism.
Production and inhibition
The enzyme is produced only in specific tissues of the male human body, namely the skin, seminal vesicles, prostate and epididymis.
Inhibition of 5-alpha reductase results in decreased production of DHT, increased levels of testosterone and possibly increased levels of estradiol. Gynecomastia is a possible side effect of 5-alpha reductase inhibition.
Pharmacology
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5-alpha-reductase inhibitor drugs are used in benign prostatic hyperplasia, prostate cancer and baldness. Finasteride inhibits the function of only one of the isoenzymes (type 2), while dutasteride inhibits both forms.
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